1. Field of the Invention
The present invention relates to a nonvolatile memory, and more particularly to a method for fabricating a nonvolatile ferroelectric memory which can prolong a life time of the memory.
2. Background of the Related Art
A ferroelectric memory, i.e., an FRAM(Ferroelectric Random Access Memory) is paid attention as a next generation memory which has a data processing rate as fast as a DRAM (Dynamic Random Access Memory) generally used as a semiconductor memory and retains the data even in turn off of power. The FRAM, having a structure almost identical to a DRAM, is a memory which employs a ferroelectric material as a capacitor material for utilizing a high residual dielectric polarization that is a characteristic of the ferroelectric material in retaining data even after removal of an electric field.
A related art method for fabricating a nonvolatile ferroelectric memory will be explained, with reference to the attached drawings. FIGS. 1a.about.1c illustrate sections showing the steps of a related art method for fabricating a capacitor of a ferroelectric material.
Referring to FIG. 1a, the steps of a related art method for fabricating a capacitor of a ferroelectric material starts with forming an insulating film 2 on a semiconductor substrate 1, and forming a bottom electrode 3 on the insulating film 2. The bottom electrode 3 is formed of Pt (Platinum). As shown in FIG. 1b, a PZT[Pb(Zr, Ti)O3]4, a ferroelectric film, is formed on the bottom electrode 3. As shown in FIG. 1c, a top electrode 5 is formed on the PZT 4. The top electrode 5 is formed of Pt.
FIG. 2 illustrates a general hysteresis loop of a ferroelectric material, and FIG. 3 illustrates P-E curves on an electric field cycle applied to the PZT used as the ferroelectric material. That is, FIG. 2 illustrates hysteresis curves of ferroelectric materials, such as PZT used for the related art memory shown in FIGS. 1a.about.1c. The ferroelectric material of PZT has a spontaneous polarization and a polarization inversion by an electric field, which are essential properties in the ferroelectric material. As shown in FIG. 2, the polarization induced by the electric field does not disappear, but a certain amount of residual dielectric polarization(+Pr and -Pr) is kept due to the spontaneous polarization even after the electric field is removed. The states of +Pr and -Pr are corresponded to 0 and 1 respectively in utilizing as a memory, and different from the DRAM, data can be kept even after removal of the electric field, thereby implementing the nonvolatile memory. In operation of the FRAM, i.e., in reading and writing a data, the two states of +Pr and -Pr should be alternated as necessary, from +Pr to -Pr or vice versa, which is called as a polarization inversion, which causes a degradation.
That is, as shown in FIG. 3, when a bipolar field is applied in succession to a ferroelectric film, such as of PZT, for causing a polarization inversion, a P(Polarization)-V(voltage) characteristic shows a gradual decrease of residual dielectric polarization of +Pr and -Pr as applied voltage cycles are increased until the memory can not serve as a memory, finally. FIG. 3 shows a sharp increase of a fatigue of the ferroelectric film when a number of the cycle is greater than 10.sup.8, which is caused by a mismatched of the ferroelectric film with an electrode material, for which researches on a new electrode material that has good match with the existing PZT is underway. FIG. 4 illustrates a graph showing fatigue of the PZT ferroelectric film, wherein a number of applied field cycles vs. residual dielectric polarization is shown in a case when PZT is used as a ferroelectric film and Pt is used for the top and bottom electrodes, wherefrom it can be known that the fatigue sharply increases at around 10.sup.8 switching cycles in the case of PZT.
However, the related art method for fabricating a nonvolatile ferroelectric memory has a problem in that the rapid fatigue of the ferroelectric film, such as PZT, started to occur when a number of re-writing which requires polarization inversion exceeds 10.sup.8 times drops a reliability of the nonvolatile ferroelectric memory.